In order to improve portability, miniaturization of radio communication terminals is being promoted in recent years. In line with this, miniaturization is also required for built-in antennas used for radio communication terminals. As a conventional built-in antenna that meets this requirement, a tabular reverse F-figured antenna is used. A built-in antenna used for a conventional radio communication terminal will be explained below.
FIG. 1 is a schematic view showing a configuration of a built-in antenna used for a conventional radio communication terminal. The elements shown in FIG. 1 are mounted in a package of a radio communication terminal, but an overall view of the radio communication terminal will be omitted for simplicity of explanation. As shown in FIG. 1, the conventional radio communication terminal is provided with base plate 1 and tabular reverse F-figured antenna 2. X, Y and Z denote their respective coordinate axes.
Furthermore, the above-described conventional built-in antenna is also used as a diversity antenna to handle variations in the radio wave reception field intensity through multi-paths. FIG. 2 is a schematic view showing a configuration of a diversity antenna used for the conventional radio communication terminal. As shown in FIG. 2, this configuration includes monopole antenna 3 as an external antenna in addition to above-described conventional tabular reverse F-figured antenna 2. Diversity reception is carried out using two antennas; tabular reverse F-figured antenna 2, which is an internal antenna, and monopole antenna 3, which is an external antenna, thereby providing stable communications.
However, in the case of the tabular reverse F-figured antenna used for the conventional radio communication terminal, tabular reverse F-figured antenna 2 operates as an exciter to excite base plate 1 rather than as an antenna. For this reason, an antenna current flows into base plate 1, and therefore the base plate becomes dominant as the antenna. As a result, tabular reverse F-figured antenna 2 used for the conventional radio communication terminal has a problem that gain is reduced due to the influence of the user's body of the above-described radio communication terminal.
Here, a specific example of the reception characteristic of tabular reverse F-figured antenna 2 used for the above-described conventional radio communication terminal will be explained with reference to FIG. 3A and FIG. 3B. FIG. 3A and FIG. 3B illustrate measured values of the reception characteristic of a tabular reverse F-figured antenna used for the conventional radio communication terminal. Here, the size of base plate 1 is assumed to be 120×36 mm and the frequency is assumed to be 2180 MHz.
First, FIG. 3A illustrates the reception characteristic of the horizontal plane (X-Y plane) in a free space of tabular reverse F-figured antenna 2 used for the conventional radio communication terminal. In this case, since base plate 1 operates as an antenna, tabular reverse F-figured antenna 2 is almost nondirectional as shown in FIG. 3A.
On the other hand, FIG. 3B illustrates the reception characteristic of the horizontal plane (X-Y plane) during a conversation of tabular reverse F-figured antenna 2 used for the conventional radio communication terminal. Here, suppose radio communication terminal is used in a condition as shown in FIG. 4. That is, radio communication terminal 4 provided with tabular reverse F-figured antenna 2 and monopole antenna 3 is used for a conversation by user 5 in the condition shown in FIG. 4.
As is apparent from FIG. 3B, the gain of tabular reverse F-figured antenna 2 is reduced during a conversation. It is obvious from a comparison between FIG. 3A and FIG. 3B that the reduction of gain of tabular reverse F-figured antenna 2 is influenced by the human body, for example, interruption of radio waves by the user's head or hands.
Then, a specific example of the radiation characteristic of tabular reverse F-figured antenna 2 used for the above-described conventional radio communication terminal will be explained with reference to FIG. 5A and FIG. 5B. FIG. 5A and FIG. 5B illustrate measured values of the radiation characteristic of the tabular reverse F-figured antenna used for the conventional radio communication terminal.
First, FIG. 5A illustrates a radiation characteristic of the horizontal plane (X-Y plane) in a free space of tabular reverse F-figured antenna 2 used for the conventional radio communication terminal. In this case, base plate 1 operates as an antenna, and therefore tabular reverse F-figured antenna 2 is almost nondirectional as shown in FIG. 5A.
On the other hand, FIG. 5B illustrates a radiation characteristic of the horizontal plane (X-Y plane) during a conversation of tabular reverse F-figured antenna 2 used for the conventional radio communication terminal. Here, suppose the radio communication terminal is used in a condition as shown in FIG. 4. As is apparent from FIG. 5B, the gain of tabular reverse F-figured antenna 2 during a conversation is reduced. It is obvious from a comparison between FIG. 5A and FIG. 5B that such a reduction of gain of tabular reverse F-figured antenna 2 is caused by the influence of the human body, for example, the influence of interception of radio waves by the user's head or hands.
As shown above, tabular reverse F-figured antenna 2 used for the above-described conventional radio communication terminal has a problem that gain is reduced by the influence of the human body.
Furthermore, with respect to a diversity antenna used for the above-described conventional radio communication terminal, operating tabular reverse F-figured antenna 2 also involves problems similar to those shown above.